For years, clinical ultrasound systems have incorporated either pulsed or continuous waves (CW) ultrasound techniques for imaging of tissue structure and flow of blood therethrough. Since the tissue investigated is a dispersive medium, signals transmitted into and thereafter reflected from tissue discontinuities suffers significiant attenuation. That is, the greater the path taken by the acoustic signal within the subject, the greater the signal attenuated and otherwise changed. Previous systems have included compensation techniques such as time-controlled gain to provide correction for the anticipated attenuation by the signal in the subject tissue. Similarly, other correction techniques have been applied with varying degrees of success. Regardless of the correction techniques used thus far, certain problems remain. Typical of these problems are the multiple reflections incurred between the surface of the specimen to be investigated, and the surface of the transducer thereof within the ultrasonic probe. Moreover, for the deeper signal penetration levels, the signal becomes excessively attenuated, often obscuring critical imaging information, and unfocussed.
One approach taken by the applicant in previously filed application Ser. No. 616,581, entitled "Tissue Signature Tracking Tranceiver," incorporates the phenomenon of tissue signature analysis, wherein the transmitted pulse returns in a frequency-skewed form as a result of the transition through the tissue medium. However effective the system described therein may be, further improvements for technical and economic reasons are desired.
While the method described in the previous disclosure provides substantial improvement in overall image quality and in overall image uniformity, the following limitations exist arising from problems of implementation and economics:
(1) When filter tracking is implemented, either filter inductance or capacitance must be modulated (e.g. saturable reactors or varactor diodes) to a very great extent because the required modulation of component values is the square of the frequency range. The resulting impedance variation is significant during the frequency tracking slew. If the percentage of frequency modulation could be reduced, the modulations of circuit component parameters could be also reduced.
(2) The low-pass and high-pass cutoff frequencies must track to maintain a constant bandwidth.
(3) The detector, regardless of its sophistication, must work with the baseband RF signal. Since the video frequency (envelope) information is so close to the lowest carrier frequency at the bottom of the echo-train, there is the practical problem of designing a broadband detector that has impressive risetime performance but still suppresses carrier ripples (e.g., second harmonic). A detector capable of meeting the fast rise-time vs. low ripple criterion must contain high-order low-pass filters that represent some cost. If greater separation between the video (envelope) information and the carrier frequency were possible, one could obtain the required detector performance with a simple first-order low-pass filter after demodulation.
(4) In the design of any practical receiver, given a simple, unshielded method of packaging, one is always faced with the question of how to design a receiver performing under extremely high gain conditions without uncontrolled feedback. If, for a given complexity and size of components, one could identify an alternative design with less crosstalk from the output back to RF front-end input, one would implement such a design for that reason alone.
(5) Frequently the ultrasound pulser-receiver is required to be used in an imaging system that operates at several different ultrasound frequencies, such as 3.5, 5, and 7.5 MHz. Up until now, either broadband receivers had been used, or receivers with simple switched fixed-frequency filters have been used. A tissue-tracking receiver, operating in several ranges, of very simple design, would be highly desirable.